1. Field of the Invention
The present invention relates to an X-ray imaging apparatus that uses a multiple X-ray source, and in particular to an apparatus that provides an X-ray tomographic image.
2. Description of the Related Art
A multiple X-ray source that uses carbon nanotubes as cold cathodes is known. This publicly-known X-ray emission apparatus forms a two-dimensional X-ray source by using multiple X-ray tubes utilizing carbon nanotubes as cathodes that emit electrons and disposing X-ray radiation windows, for collecting the X-rays from the X-ray tube, in a two-dimensional arrangement. The X-rays emitted from the two-dimensional X-ray source of the X-ray emission apparatus pass through a subject and are irradiated upon an X-ray image detector. The X-ray image detector generates an image signal of the X-ray image based on the intensity of the irradiated X-rays. A collimater, in which capillaries are arranged two-dimensionally in a sieve-like form, is disposed between the two-dimensional X-ray source of the X-ray emission apparatus and the subject, so that the axial direction of the capillaries follows the same direction as the direction between the two-dimensional X-ray source and the subject (see Japanese Patent Laid-Open No. 2004-089445 (called “Patent Document 1” hereinafter)).
Meanwhile, a technique whereby a tomographic image of a subject is calculated based on transmitted X-ray images created using multiple X-ray sources is known. In this publicly-known technique, a radiation source that irradiates a target surface with an electron beam, causing the emission of X-rays, and forms the X-rays into beam form by passing them through a collimater hole, is used. Many collimater holes are provided on the surface. The X-rays that pass through the subject are sequentially detected by a radiation detector, while scanning the electron beam and sequentially switching the collimater hole. A transmitted image forming means obtains transmitted image information based on the detection signals from the radiation detector that hold subject image information for each pixel point. Because the X-rays enter into the radiation detector directly from the collimater holes, and almost no scattering rays enter, no scattering ray information is included in the transmitted image information; thus, three-dimensional image information that includes no scattering ray information can be obtained (see Japanese Patent Laid-Open No. 2000-060835 (hereinafter called “Patent Document 2”)).
The X-ray imaging system disclosed in Patent Document 2 is formed in a quadrangular cone shape, as viewed in the direction extending from the focal point to the detector. The detection target is therefore captured over a wide range in regions close to the detector, and over a narrow range in regions far from the detector. When this phenomenon is observed in the vicinity of the X-ray sources, projection data of the detection target cannot be acquired in the pitch interval between multiple X-ray sources. In other words, data missing regions arise in the pitch areas between X-ray sources. This problem becomes more marked the larger the enlargement rate of the imaging system is.
Another problem is the reciprocal relationship between scattering ray removal and X-ray usage efficiency. In conventional X-ray imaging, the cone angles of the quadrangular cone-shape formed by the X-ray beam are enlarged, increasing the X-ray usage efficiency, and a large subject region is captured at one time. However, when a large region is irradiated with X-rays at one time, the resolution of the image decreases due to scattering rays. Although a scattering ray suppression grid can be used to suppress such scattering rays, the scattering ray suppression grid is disadvantageous because it dampens valid straight rays as well. Patent Document 2 therefore reduces the cone angles, which reduces the scattering rays, but reducing the cone angles also causes a drop in the X-ray usage efficiency.